Anhydrous borax is a sponge for moisture. So unless one plans to use it all quickly or lives in a dry climate (Portland definitely doesn't count), I'd recommend that one save their money and get a box of "20 mule team borax." If one wants to make it anhydrous, pour a 1/4 to 1/2" layer in a pie pan and heat to 400-500F for 30 to 60 minutes. Even that is not needed, sprinkling wet borax on red hot metal will dry it instantly, though with some bubbling.

I like the borax and iron or iron oxide combo, because it creates a thin layer of molten iron between the pieces being welded which in turn makes it easier to weld them.